As to a disk apparatus using such an optical disk or a magneto-optical disk, mounted on a drive chassis are a spindle motor that rotates a turntable, and an optical pickup unit that is a record-playback unit, whereby an optical disk that is a discoid recording medium placed on the turntable is rotationally driven by the spindle motor at high speed. Under this situation, the optical pickup unit performs, while tracking the optical disk, recording digital information onto the optical disk, or playing back recorded digital-information therefrom.
When an optical disk rotates at high speed, due to eccentricity of the center of gravity of the optical disk itself, vibrations are generated in directions perpendicular to the axial direction of the spindle motor. The vibrations are transferred to the optical pickup unit by way of the drive chassis, resulting in causing vibrations to the optical pickup unit as well.
On this occasion, when phase deviation is generated between vibrations of the spindle motor and vibrations of the optical pickup unit, bad influence is exerted on a tracking servomechanism of a light beam focused by an objective lens of the optical pickup unit, resulting in an occurrence of errors in writing digital information onto and/or reading that from the optical disk.
In consequence, generation of vibrations of a drive chassis causes a critical problem in a high-density optical disk such as DVD (digital versatile disc) because of significant degradation in recording and/or playing back performance for digital information.
To this end, in order to curb vibrations of a drive chassis that are caused by eccentricity of the gravity-center of an optical disk itself or by vibrations from outside in a conventional disk apparatus, a vibration-damping mechanism that is referred to as “dynamic damper” has been adopted to be mounted.
As to the vibration-damping mechanism, a weight plate is mounted by way of a plurality of elastic members onto the drive chassis for which the vibrations are required to be curbed, and the weight plate is made floatingly and movably supported as far as it counteracts the elastic force attributed to the elastic members.
Excitation force according to the rotation of an optical disk having eccentricity of the center of gravity acts in the directions parallel to the optical-disk's main surface having the rotational center of the optical disk in its plane of revolution. Therefore, in regard to vibrations of the drive chassis in tracking directions caused by the excitation force, causing the weight plate to vibrate in an orientation opposing to that of the vibrations of the drive chassis makes it possible to cancel out, by the mass of the weight plate, the vibrations of the drive chassis in the tracking directions.
Similarly, in regard to vibrations of the drive chassis in directions perpendicular to the tracking directions in the plane of revolution of the optical disk, it is possible to cancel out such vibrations of the drive chassis by causing the weight plate to vibrate in an orientation opposing to that of the vibrations.
To this end, the weight plate constituting a conventional vibration-damping mechanism is so arranged as to curb the vibrations of the drive chassis in the two mutually orthogonal directions parallel to the main surface of an optical disk, under the condition that a spring constant of the elastic members for the vibration-damping mechanism and the mass of the weight plate are set so that the vibration-damping mechanism's natural-vibration frequencies of natural-vibration modes each in two directions in the plane of revolution of the optical disk are made coincident with a so-called vibration-damping frequency that is the vibration frequency of the drive chassis having the vibrations which should be curbed (for example, refer to Patent Document 1).
[Patent Document 1] Japanese Patent Application Publication No. 2003-7033 (Pages 2 and 3, FIG. 27).